Metathetic processes of the kind specified occur in aqueous solution under conditions in which the forward reaction is favored. Where one of the products MY and HX is water insoluble or volatile and accordingly precipitates or evaporates from the solution, the equilibrium of the reaction is constantly shifted from left to right leading to a continuous forward reaction. Depending on whether the desired product is the salt MY or the acid HX or possibly the two of them, the reaction solution and/or the precipitate must be worked up for the recovery of the desired product therefrom. Where, however, both the product MY and HX are water soluble, a steady state will be reached after a while in which the forward and backward reactions are in equilibrium and some kind of intervention is accordingly required for inducing continuation of the forward reaction. In the following, a metathetic reaction of the kind specified in which one of the two products is of limited water solubility or higher volatility and accordingly separates from the aqueous reaction mixture in the course of the reaction, will be referred to as spontaneous forward reaction while a reaction in which both products are water soluble and intervention is required for shifting the equilibrium from left to right and thereby induce continuation of the forward reaction, will be referred to as an induced forward reaction.
A typical example of metathetic processes with spontaneous forward reactions are the so-called wet process production of phosphoric acid by reaction of calcium phosphate with sulfuric acid, and the recovery of citric acid from a fermentation liquor by the so-called liming/acidulation process, which may be described, respectively, by the following Equations II and III EQU Ca.sub.3 (PO.sub.4).sub.2 +3H.sub.2 SO.sub.4 =3CaSO.sub.4 +2H.sub.3 PO.sub.4II EQU Ca.sub.3 (Cit).sub.2 +3H.sub.2 SO.sub.4 =3CaSO.sub.4 +2H.sub.3 PO.sub.4III
Typical examples for metathetic processes with induced forward reactions are production of the multi-nutrient fertilizer potassium nitrate from potassium chloride and nitric acid, in which the product of interest is a salt; and the conversion of ammonium lactate, which is a direct product of lactic acid fermentation, into free lactic acid by reaction with sulfuric acid, the product of interest here being an acid. These two processes are described, respectively, by the following equations IV and V EQU KCl+HNO.sub.3 =KNO.sub.3 +HCl IV EQU 2NH.sub.4 La+H.sub.2 SO.sub.4 =(NH.sub.4).sub.2 SO.sub.4 +2HLaV
where La is lactate.
In reaction IV the main product is KNO.sub.3 and in reaction V the product of interest is HLa. In both reactions, the products are water soluble which makes it necessary to induce the forward reaction.
Solvent extraction is commonly applied for the inducement of forward reactions by product separation in metathetic processes. Alkanols, ethers, esters, ketones and other oxygen-carrying, water-immiscible compounds are well known acid extractants operating through salvation of the acid (solvating extractants). Acid binding in such extracting operations is relatively weak and as a result, the solvating extractants are effective only at relatively high acid activities, and by themselves are, as a rule, not capable of providing the driving force required for the inducement of the forward reaction. While the reaction of potassium chloride with sulfuric acid to form potassium sulfate and hydrochloride acid can be facilitated by acid extraction with alkanols, at the acidity levels required for efficient extraction, the acidic salt KHSO.sub.4 is formed rather than K.sub.2 SO.sub.4. In addition, Cl.sup.- /SO.sub.4.sup.2- extraction selectively is low which entails that for effective separation of HCl from KHSO.sub.4 many extraction stages are required. Quite generally, due to low binding energy, solvating extractants are not effective for inducement of the displacement of a strong acid by a weak one.
Amine based extractants are much stronger acid binders and thus effective also at low acidities. They are accordingly capable of providing acids, the driving force for metathetic processes in which weak acids react with salts of strong acids, and they are accordingly widely used for the withdrawal of product acids and the inducement of the forward reaction in a metathetic process.
Amine based extractants have, as a rule, a high anion/anion selectivity and thus provide for good separation from each other of the anions in the system and thereby for higher yields and higher purities of the products. This high selectivity may, however, become counter productive and induce backward reactions in cases where the starting acid HY is preferentially extracted and this indeed is the case in the process according to equation IV above in that from a solution containing the ions Cl.sup.-, NO.sub.3.sup.-, H.sup.+ and K.sup.+, HNO.sub.3 is extracted preferentially by amine based extractants and it is for this reason that amine based extractants cannot be used in the case of equation IV and less attractive solvent extractants or other means are required. Thus, in industrial processes for KNO.sub.3 production, the by-product HCl is removed through chemical conversion. In one process KCl and HNO.sub.3 are reacted at temperature, concentration and acidity at which oxidation/reduction reactions take place whereby chlorine, nitrogen oxides and other products are formed. Complex separations procedures, NOx conversion to nitric acid and operation at highly corrosive conditions, all of which are required for inducing the forward reaction, result in low profitability.
KNO.sub.3 production from KCl and NHO.sub.3 can also be mediated by ion exchangers. Passage of a KCl solution through a cation exchanger in its acid form results in loading the resin with K.sup.+ ions and formation of an HCl solution. The cation exchanger is then eluted with an HNO.sub.3 solution to produce KNO.sub.3 and regenerate the cation exchanger. Alternatively, it is possible to transfer KCl solution through NO.sub.3.sup.- loaded anion exchanger which is then regenerated by HNO.sub.3.
Likewise, solvent extraction with an amine based extractant also induces backward reaction in case of equation V above in that, from an aqueous solution containing La.sup.-, SO.sub.4.sup.-, HSO.sub.4.sup.-, H.sup.+ and NH.sub.4.sup.+, HSO.sub.4 is extracted preferentially whereby the backward reaction is induced. Solvating extractants have the required selectivity and extract lactic acid from the solution without, however, yielding the advantages afforded by amine based extractants. Accordingly, it has been proposed to recover lactic acid from a fermentation broth by processes other than a metathetic reaction. Thus, EP 0517242 (Mantovani et al.) describes a process by which the lactate values of a nearly neutral lactic acid fermentation liquor are first separated by means of a carbonate loaded anion exchanger, which is then eluted by ammonium carbonate to form an aqueous ammonium lactate and an ammonium carbonate solution. The ammonium carbonate in solution is decomposed thermally and the ammonium lactate is passed through a cation exchanger in its acid form to yield a lactic acid solution and an ammonium loaded cation exchanger, which latter is regenerated by a mineral acid solution such as aqueous HCl. This process thus involves two ion exchange operations, the first of which provides for purification while the second mediates the metathetic reaction of equation VI EQU NH.sub.4 La+HCl=NH.sub.4 Cl+HLa VI
In order to avoid product or by-product contamination and low yield, large volumes of ion exchanger and marked dilution are applied in both the above ion exchange operation. Further dilution is imposed by unavoidable washing of the ion exchanger resin after each stage, all of which results in a dilute product solution. It is thus evident that these and similar processes are cumbersome and not practical for industrial application.
It is the object of the present invention to provide in an acid-salt metathetic process, a new method for inducing forward reactions.